The present invention concerns solid-state meters for measuring AC electrical energy, or power, consumed by a load. In particular, the invention concerns watthour and varhour meters, or the like, utilizing potential and current transformers. More specifically, the invention concerns an improvement in power factor matching of the potential transformer to the current transformer.
Electronic or solid-state watthour (or varhour) meters have been known since the early 1970's. In meters of this type, the load current and line voltage are sensed and corresponding current and voltage signals are input into a measurement circuit. These current and voltage signals are processed by the measurement circuit to produce an output signal indicative of the AC electrical energy consumption by the load, which signal is then supplied to a register for further processing and display.
An example of one type of solid-state watthour meter is shown in FIG. 1. More particularly, the meter 10 includes an array of AC quantity transducers 12 which produce voltage and current signals for each phase of a polyphase system. These voltage and current signals are fed to measurement circuit means 14, which in the illustrated embodiment is a watt transducer chip. In one specific embodiment, the measurement circuit 14 comprises a custom integrated circuit which performs Time Division Multiplication of the voltage and current signals by way of a mark space oscillator and an amplitude modulator applied to the respective voltage and current signals. An appropriate measurement circuit means or watt transducer chip can be found in the Landis & Gyr SSM20 meter. Some details of this chip can be found in U.S. Pat. No. 4,896,106, at columns 3-5, which description is incorporated herein by reference.
The measurement circuit means 14 generates AC energy consumption data 16 which is fed to a register 18. The register can include a microprocessor which manipulates the energy or power consumption data to produce, for example, time of use information. The register 18 includes a display, a memory, and an option board in one specific embodiment. A power supply transformer 20 provides DC power to both the measurement circuit means 14 and the register 18.
In the illustrated solid-state watthour meter, the array of AC quantity transducers 12 include a number of potential transformers 22.sub.1-3 and a number of current transformers 23.sub.1-3. One each of the potential and current transformers is provided for each phase of a three phase AC power supply system in the illustrated embodiment. The solid state watthour meter 10 shown in FIG. 1 as thus far described is of a known construction.
In any AC energy consumption or power metering, it is essential to have a correct registration between the voltage and current signals provided to the measurement circuit. When the registration between these voltage and current signals deviates from an expected value, the calculations of the AC energy usage, whether it be watthours or varhours, is compromised. In any AC metering arrangement, the AC power drawn by the load can vary at different load power factors. This variation in the load power factor can cause a deviation between the measured and displayed energy consumption value and the actual AC energy consumption by the load. It is known that the power factor is the ratio of the active power to the apparent power (which is the square root sum of the squares of the active and reactive power). Consequently, the power factor is a function of the phase angle difference between the AC current and voltage provided by the AC source.
In the late 1800's, it was determined that in order for a meter to have a correct registration with varying load power factor, the potential transformer flux must lag the current transformer flux by exactly 90.degree. when the load on the meter is at the unity power factor. This 90.degree. relationship at unity power factor was determined to be necessary to maintain a driving force on the meter disc proportional to the power at any load power factor value. In inductive meters of the prior art, a phasing band or coil was provided around the core of the center leg of the potential coil to cause the potential transformer flux to lag the current transformer flux by 90.degree. . Shifting the current transformer flux towards the potential transformer flux until the power factor angle was exactly 90.degree. has been accomplished by inserting a closed "figure 8" circuit loop on the current magnets to produce a voltage in that loop, which causes current to flow in the loop thereby creating a counter-acting flux field.
Alternatively, a lag plate has been moveably positioned on the pole piece of the potential transformer to provide adjustable phase compensation of the voltage signal wit respect to the current signal. Many modern meters use a fixed lag plate operating on the potential flux with the phase compensation being permanently made by the manufacturer at the factory.
As solid-state meters replace the standard inductive meter, the concern over maintaining a proper meter disc driving force diminishes. However, the importance of power factor matching to meter registration has not diminished. The meter power factor defines the relationship between energy usage measurement values such as watts, vars and volt-amperes. It is therefore necessary to control and match the meter power factor to minimize any error in the metered AC energy consumption versus the true energy consumed by the load.